The availability of satellite and microwave communications links, together with recent advances in electronics technology and signal processing hardware, have opened the door to the establishment of controlled access transmission networks. While the reduced cost of earth station receiving equipment brings the investment for such networks to a reasonable level, it also makes off-air piracy viable. Laws governing off-air reception by anyone willing to invest in the necessary equipment are either nonexistent or difficult to enforce. There is therefore a need for a method of scrambling television signals whereby to establish high security in such networks. Coupled with this need is the need to transmit the scrambled signals through existing distribution channels. Thus, parameters such as synchronizing information, finite bandwidth restriction and characteristic distortion of a link must be taken into account.
In any scrambling system, since the transmitted audio and video and data information has been scrambled to prevent its unauthorized use, it is necessary to provide descrambling control information to allow reconstruction of the transmitted signals by the authorized users. The security of this control information is therefore crucial to the security of the whole system.
Various analog scrambling methods are known and in current use. The methods employ, for example, sync suppression, video inversion or interfering carrier, and achieve scrambling by changing the transmission format of the video information in such a way that a standard receiver will not be able to reconstruct the image. The security provided by these known methods is quite limited, however. Since they rely extensively on hardware for decoding, they can be considered to be fixed algorithms in the sense that a one-time addition to, or modification of, an existing equipment will result in violation of the security. When the security is thus violated it can only be regained by replacement of the scrambling and descrambling equipment throughout the system.